Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
CH 2: Scientific Measurement Renee Y. Becker CHM 1025 Valencia Community College 1 Measurements • Measurement – A number with an attached unit Examples: 15 inches 3 cups 36 cm Every measurement must include units!! 2 Example 1: Measurements In the following, what are the measurements and what are the units? a) 12 trees b) 2.3 mm c) 100 ¢ 3 Measurements • Instrument – A device for recording a measurement • Examples: Ruler (length), electronic balance (mass), Graduated cylinder (volume) 4 Uncertainty • An instrument may give a very sensitive reading, but EVERY measurement has UNCERTAINTY • No measurement instrument is perfect and neither is the person using it 5 Example 2: Length Measurements • If we need to measure the length of this candycane, which ruler should we use? Why? 6 Mass • Mass – Measure of the amount of matter it possesses – Measured by a balance – Not affected by gravity – Typical units: kilogram (kg), gram (g), pound (lb), ounce (oz) • Weight – Force exerted by gravity on an object 7 Balances 8 Example 3: Mass • Would you have the same mass on the moon as on Earth? Why? • Would you have the same weight on the moon as on Earth? Why? 9 Volume • Volume – The amount of space occupied by a solid, gas, or liquid – Graduated cylinder, pipet, buret, volumetric flask, syringe – Typical units: milliliter (mL), Liter (L), centimeter cubed (cm3), quart (qt), gallon (gal), 10 11 Buret 12 Accuracy, Precision, and Significant Figures in Measurement • Accuracy is how close to the true value a given measurement is. • Precision is how well a number of independent measurements agree with one another. 13 Accuracy, Precision, and Significant Figures in Measurement • Significant Figures are the total number of digits in the measurement. • The results of calculations are only as reliable as the least precise measurement!! • Rules exist to govern the use of significant figures after the measurements have been made. 14 Accuracy, Precision, and Significant Figures in Measurement • Rules for Significant Figures: – Zeros in the middle of a number are significant – Zeros at the beginning of a number are not significant – Zeros at the end of a number and following a period are significant – Zeros at the end of a number and before a period may or may not be significant. 15 Example 4: Significant Figures How many Sig. Figs ? a) 0.000459 b) 12.36 c) 36,450 d) 8.005 e) 28.050 16 Accuracy, Precision, and Significant Figures in Measurement • Rules for Calculating Numbers: – During multiplication or division, the answer can’t have more sig figs than any of the original numbers. 17 Example 5: Significant Figures a) 238.5 x 79 = b) 12 / 0.1272 = c) 0.2895 x 0.29 = d) 32.567 / 22.98 = 18 Accuracy, Precision, and Significant Figures in Measurement -During addition or subtraction, the answer can’t have more digits to the right of the decimal point than any of the original numbers. 19 Example 6: Significant Figures a) 238.5 + 79 = b) 12.3 - 0.1272 = c) 0.2895 + 0.29 = d) 32.567 - 22.98 = 20 Accuracy, Precision, and Significant Figures in Measurement • Rules for Rounding Numbers: – If the first digit removed is less than 5 - round down – If the first digit removed is greater than 5 - round up – If the first digit removed is 5 and following numbers are nonzero - round up – If the first digit removed is 5 and following numbers are zero - round down – Only final answers are rounded off, do not round intermediate calculations 21 Example 7: Rounding Round off each of the following measurements (a) 3.774499 L to four significant figures (b) 255.0974 K to three significant figures (c) 55.265 kg to four significant figures 22 Example 8: Accuracy & Precision Which of the following is precise but not accurate? 23 Scientific Notation • Changing numbers into scientific notation – Large # to small # – Moving decimal place to left, positive exponent 123,987 = 1.23987 x 105 – Small # to large # – Moving decimal place to right, negative exponent 0.000239 = 2.39 x 10-4 • Correct scientific notation: #.#### x 10n 24 Scientific Notation How to put into calculator?? 25 Example 9: Scientific Notation Put into or take out of scientific notation a) 87542 b) 2.1956 x 10-3 c) 0.784 d) 2.78 x 106 e) 92000 26 Significant Figures • When we count something, it is an exact number. – It has an infinte number of significant figures 27